This invention relates to a multi-fuel, combined cycle power plant.
For many years, large scale gas turbine based stationary power plants fueled by conventional gas or liquid fuels, such as natural gas, liquefied petroleum gas (LPG), etc., have been used by utilities for peaking purposes because of the fast, on-line response of a gas turbine, and the relatively low cost of large gas turbine plants. More recently, combined cycle power plants have been constructed to increase the efficiency of the system thus enabling the gas turbine to be incorporated into power plants that provide base load electric power. Such power plants include a waste heat boiler that extracts heat from the exhaust gases of the gas turbine for generating steam that drives a steam turbine and produces additional power. Furthermore, sometimes supplementary duct firing has been used as an addition to the waste heat boiler.
At the same time, efforts have been made to utilize less expensive, and more abundant low heat content solid fuels, such as low quality coal, oil shale, bituminous phosphates, biomass, etc., in base-line power plants. Conventionally, power plants using such low heat content fuels have employed fluidized bed combustors that include integral heat transfer elements for generating steam. Examples of such power plants are disclosed in application Ser. No. 08/747,400 filed Nov. 12, 1996, the disclosure of which is hereby incorporated by reference.
It is an object of the present invention to provide a new and improved combined cycle power plant operating on low cost solid fuel in which several of the disadvantages outlined are reduced or substantially overcome.
A power plant according to the present invention includes a prime mover for burning fuel and producing work and hot exhaust gases, and heat exchanger apparatus that is responsive to the hot exhaust gases and to pre-heated steam condensate for converting the latter into a heated water product, e.g., high pressure steam, and for producing heat depleted exhaust gases. A combustor, preferably, a fluidized bed combustor, and part of a combustor-heat exchanger apparatus, is responsive to burning fuel, such as low cost solid fuel, for supplying additional heat to a product of the heat exchanger apparatus, and producing combustion products. The combustion products are supplied to a superheater for producing superheated, high pressure steam from the high pressure steam which is a product of the heat exchanger apparatus. A high pressure steam turbine expands the superheated high pressure steam and produces power and exhaust steam that is condensed to steam condensate in a condenser. Finally, the power plant includes an economizer that is responsive to heat-depleted combustion products and to the steam condensate for producing the pre-heated steam condensate used by the heat exchanger apparatus.
One form of the prime mover includes a gas turbine unit having a compressor for producing compressed air, a burner for combusting fuel and heating said compressed air to produce heated gases, and a turbine for expanding the heated compressed gases and driving the compressor.
In one embodiment of the present invention, the combustor receives the heat depleted exhaust gases produced by the heat exchanger apparatus. When the combustor is a fluidized bed combustor, the heat depleted exhaust gases serve to fluidize said combustor.
Preferably, the heat exchanger apparatus includes a first vaporizer responsive to the hot exhaust gases from the gas turbine, and to a portion of the pre-heated condensate for converting the latter into high pressure steam and producing the heat depleted exhaust gases. The additional heat supplied by the combustor is applied to the superheater containing the steam produced by the first vaporizer for converting the latter to superheated high pressure steam supplied to the high pressure steam turbine.
The power plant of the present invention may also include a second vaporizer interposed between the superheater and the economizer for extracting heat from the heat depleted combustion products supplied from the superheater and vaporizing an additional portion of the pre-heated condensate and producing further high pressure steam which is also supplied to the superheater.